Synchronous motors are AC motors which rotate at a fixed rate, known as synchronous rate, which depends on the frequency of the supply voltage and on the number of poles of the motor.
Synchronous motors are generally substantially constituted by a rotor, having a ring of magnetic poles excited with direct current (inductor), and by a stator, constituted by a laminated magnetic core; the armature winding is located in the internal peripheral region of said core, within grooves formed therein, and is connected to the AC supply line by means of terminals.
In the particular case of permanent-magnet motors, the inductor is constituted by a permanent magnet, generally made of alnico or magnetized ferrite, instead of by poles excited with DC current.
Small permanent-magnet synchronous motors are currently in widespread use owing to their simplicity and low cost and are usually used for low power levels.
The synchronous motor, however, entails some drawbacks. First of all, it is not possible to predict the starting rotation direction; moreover, the static torque offered is substantially weak, making startup sometimes difficult even in the presence of relatively small loads.
Various devices have been devised in order to obviate this drawback, but they are particularly complicated and expensive and therefore cannot be used conveniently in motors which must have a modest cost, such as for example those designed for pumps or similar devices.
In many cases, the use of these systems entails structural modifications to the motor, making their use impossible in normal mass-production.